WASHINGTON — President Obama’s choice for a powerful appeals court appointment is in peril from both the left and the right, highlighting how the fraught politics of an election year are threatening the president’s agenda even among his allies on Capitol Hill.

The nomination of David Barron, who was a Justice Department lawyer at the start of the administration and is now a Harvard Law School professor, is mired in a maw of contentious issues. Republicans object to what they say are his radically liberal views on the Constitution. Democrats in conservative-leaning states, especially those who are up for re-election, are wary that a vote for him might backfire with voters at home. And members of both parties say they are disturbed by Mr. Barron’s authorship of legal memos that justified the United States’ killing of an American citizen overseas with a drone.

The American Civil Liberties Union wrote to all 100 senators on Monday urging them to put off a vote on Mr. Barron’s confirmation until the White House allowed them to read all of his writings on the drone program.

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Senator Rand Paul, Republican of Kentucky, wants the administration to release one of Mr. Barron’s memos.CreditGabriella Demczuk/The New York Times

The White House said Tuesday that it would allow senators to see one of those memos in a classified setting, the one that dealt with killing the American-born terrorist leader Anwar al-Awlaki. But it remained unclear if that would be enough to sufficiently address the concerns that many senators have expressed.

Mr. Barron, chosen by the president last year for a vacancy on the United States Court of Appeals for the First Circuit, needs 51 votes, meaning only a handful of Democrats, who control 55 seats, could defect. The majority leader, Harry Reid, has not decided whether he will proceed with a vote given the uncertainty, a spokesman said Monday.

The A.C.L.U.’s objections, along with the announcement by Senator Rand Paul, Republican of Kentucky, that he would use his power to slow down the confirmation unless the administration released one of the legal memos written by Mr. Barron, raised fresh questions on Capitol Hill on Monday about whether the nomination would survive.

Last year, Mr. Paul delivered a 13-hour speech on the floor of the Senate demanding that the White House disclose more information about its legal justification for drone attacks.

Mr. Barron’s nomination never sat particularly well with conservatives. During his confirmation hearing, Senator Ted Cruz of Texas summed up the difficulty many on the right had supporting him, saying he doubted whether someone with such “a progressive view of the Constitution” could serve on the federal bench. But Democrats, initially at least, voiced little objection, and he received the support of his home state senator, Elizabeth Warren of Massachusetts.

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Senator Mark Begich, Democrat of Alaska, is up for re-election and said to be wavering on Mr. Barron’s nomination as a judge.CreditGabriella Demczuk/The New York Times

The wariness of Democrats, and the willingness of some to buck their president, began to surface publicly on Monday.

Two Democrats who are up for re-election in states where Republicans have a political edge — Mark Begich of Alaska and Mary L. Landrieu of Louisiana — are said to be unsure if they will vote yes on Mr. Barron.

A court has ordered the administration to release some of Mr. Barron’s legal work as part of a Freedom of Information Act lawsuit. But White House lawyers have not done so while they weigh whether to appeal. Senator Mark Udall, a Colorado Democrat who is in a tight race, said Monday that he would vote no unless the White House released what the court ordered.

“Unless the White House complies, I cannot support David Barron’s nomination,” he said.

Senator Ron Wyden, Democrat of Oregon and one of the senators who have voiced deep concern over the secrecy of the administration’s drone policy, said Monday that he had agreed that the vote on Mr. Barron should not move forward until senators could see more of the secret legal opinions that the nominee produced at the Justice Department.

Mr. Wyden added that he was also not committed to voting yes.

“Certainly the opinion would not be something I would have written. The question is: Is it disqualifying,” he said, adding that the administration should start the process of releasing the memos. “It needs to be addressed before a vote.”

A version of this article appears in print on , on Page A14 of the New York edition with the headline: Judicial Nominee’s Memos on Drones Stirring Bipartisan Concern in the Senate. Order Reprints | Today’s Paper | Subscribe